Leadless solid electrolyte tantalum capacitor

ABSTRACT

A SOLID ELECTROLYTE TANTALUM CAPACITOR IS PROVIDED WHEREIN THE WIRE CONNECTION TO THE TANTALUM SINTERED PELLET IS ELIMINATED. A NONPOROUS TANTALUM PAD IS PRESSED OR WELDED TO THE PELLET BODY AND, AFTER ANODIZATION, A ONOWETTING, HIGH-TEMPERATURE RESISTANCE COATING IS APPLIED OVER THE OXIDE FILM COVERING THE PAD. THE COATING ACTS TO MASK THE PAD FROM SUBSEQUENT SEMICONDUCTOR DEPOSITION. AN ANODE CONNECTION CAN THEN BE MADE EITHER BY EXPOSING THE SURFACE OF THE TANTALUM PAD OR BY WELDING THROUGH THE MASKING AND UNDERLYING OXIDE TO THE PAD SURFACE.

Inventor Alan G. Cooper 3.054029 9/1962 Wagner et al 317/230 NorthAdams. Mass. 3.243316 3/1966 O'Nan et a1. 317/230 Appl.No. 814.950 3.277349 10/1966 Hyland etal... 317/230 ggf Primary Examiner-James D Kallam eAttorneys-Connolly and Hutz, Vincent H. Sweeney, James Assign SpragueElectric Company Paul O'Sullivan and David R. Thornton North Adams.Mass.

LEADLESS SOLID ELECTROLYTE TANTALUM CAPACITOR 5 claimsz Drawing FigsABSTRACT A solid electrolyte tantalum capacitor is provided wherein thewire connection to the tantalum slntered U.S. 317/230, pelletiseliminami 29/5 70 A nonporous tantalum pad is pressed or welded to thepellet 8 9/05 body and, after anodization, a nonwetting,high-temperature Field 317/230 resistance coating is applied over theoxide film covering the i 233; 29/5 70 pad. The coating acts to mask thepad from subsequent References Ci'ed semiconductor deposition. An anodeconnection can then be 7 made either by exposing the surface of thetantalum pad or by UNITED STATES PATENTS welding through the masking andunderlying oxide to the pad 2/1960 Wagner............... 317/230surface.

United States Patent LEADLESS sour) ELECTROLYTE TANTALUM cxmcrroaBACKGROUND OF THE INVENTION This invention relates to solid electrolytetantalum capacitors and particularly to such a capacitor wherein theconventional riser wire connection to the anode is eliminated.

The above capacitors are finding increasing use in applications whereinextremely high frequency signals (over 0.5 mHz.) are encountered, i.e.,digital computers. At these frequencies, the self-inductance of thecapacitor creates an inductive reactance which limits capacitoroperation at these frequencies. A significant portion of this inductanceis created by the lead wires to the capacitor, primarily the anodelead-in wire or riser. This anode connection is especially troublesomesince it requires that a tantalum riser be welded or pressed into theanode followed'by the welding, to the riser, of the positive wireconnection. Attempts to eliminate the riser connections heretofore beenunsuccessful since the sintered body, after anodization and electrolytedeposition, will not support or hold" a direct connection made throughthe formed oxide and electrolyte layers. Also, the close proximity ofraw metal and semiconductor create a condition wherein shorts are likelyto occur.

It is therefore the object of the present invention to provide a solidelectrolyte capacitor whose inductive reactance at high frequencies isgreatly reduced. 7

It is a further object to provide such a capacitor having a novel anodeconstruction which permits direct connection of a positive lead to theanode after anodization and electrolyte deposition have been completedthereby eliminating the usual riser connection heretofore required.

SUMMARY OF THE INVENTION Broadly, this invention concerns leadlesstantalum capacitors and more particularly to the sintered anode pelletof a solid electrolyte tantalum capacitor which has provided on aportion thereof, a nonporous tantalum pad of appropriate thickness towhich a direct lead connection can be made after processing steps arecompleted.

This pad is attached by welding or pressing and sintering, a tantalumpad of appropriate thickness to the powder pellet surface. The pad cantake the form of a bar or a disc; it can be any desired size so long asa lead connection can be made to it and its location can be either onthe end or lateral surface of the pellet. The pad can project above thesurface of the pellet or can be designed to be flush with the surface.The pad surface is protected from subsequent electrolyte processing by acoating of a nonwetting agent capable of withstanding the associatedhigh temperatures. Upon removal of this coating and the underlyingoxide, the pad surface can be used as an anode termination by anydesired means.

DESCRIPTION Description OF THE DRAWING FIG. 1 is a perspective view of asolid electrolyte capacitor having a contact area according to theinvention;

FIG. 2 is a cross-sectional view through line 2-2 of the capacitor ofFIG. 1.

DETAILED DESCRIPTION OF THE INVENTION Referring to FIG. 1, there isshown capacitor 10 having a composite anode 11 comprising anode pellet12 and bar 13. An oxide coating 14 forms the dielectric layer of thecapacitor. A coating 15 of suitable solid electrolyte such as manganesedioxide overlies dielectric layer 14. The cathode is provided byapplying a silver coating 16 over electrolyte coating 15. Nonwetting,heat-resistant segments 17, (portions of coating 18 shown partiallydotted) overlie the portions of dielectric layer 14 formed on bar 13.

The capacitor is formed in the following manner. Composite anode 11 isprepared by spot welding or pressing and sintering together tantalum bar13 of a suitable thickness and tantalum pellet 12. A dielectric layer isthen formed over the surface of the composite anode. A Teflon (i.e.polytetrafluoroethylene coating 18, approximately 0.003 in. thick, ispainted over the portions of the dielectric layer formed on the barsurface and the capacitor processing is completed by deposition of alayer of manganese dioxide. The MnO does not adhere to the Tefloncoating; hence, layer 15 is only formed around pellet 12 as shown. Aportion of the Teflon and dielectric on the bar surface is abraded awayto expose the bar tantalum surface. This action results in the formationof segments 17 and leaves a remaining dielectric layer 14, a portion ofwhich serves to insulate the manganese dioxide layer 15 from the exposedtantalum metal. Layer 1.6 is then applied over a portion of theelectrolyte coating to serve as the cathode termination. While Teflon isused to form coating 18 in the preferred embodiment, any of thepolymerized fluorocarbons may be used. The required properties of thematerial used are that it be nonwetting and that it be heat-resistant upto the temperature required during processing (up to 400 C).

The exposed surface of the tantalum bar is now available as an anodetermination. For example, a silver coating can be applied over theexposed surface and the desired connection soldered in place.Alternatively, nickel or other conductive metals can be attached to thesurface by parallel gap or indirect resistance welding. In the latterinstance, it is possible to weld directly through the Teflon and oxidecoating; hence, the abrading mentioned above is not required.

Although only one embodiment of the invention has been shown, otherembodiments may obviously be provided. The bar may e located over otherportions of the capacitor and may be larger or smaller than indicated.The capacitor section may be cylindrical rather than rectangular with anendmounted tantalum disc. And while the preferred embodiment utilizes atantalum anode, the invention should be understood as not beingrestricted thereto.' The invention is to be understood to include othervalve-forming metals such as aluminum, zirconium, and niobium and whilethe preferred electrolyte cited is manganese dioxide, any of the higheroxides of manganese, lead and nickel can be used as well as the organicsemiconductors such as the quaternary ammonium complex salts.

I claim:

1. A solid electrolyte capacitor comprising an anode consisting of aporous valve-metal section and a nonporous valvemetal section integrallyconnected together forming a unitary anode singular anodic oxide; adielectric layer formed over the surface of said porous and nonporoussections of said anode; a coating of a nonwetting, heat-resistant,electrically insulating material overlying the portion of saiddielectric layer formed on said nonporous section; a solid electrolyteoverlying the portion of the dielectric layer not covered by saidelectrically insulating coating; and a conductive cathode layeroverlying at least a portion of said electrolyte.

2. The capacitor of claim 1 wherein said valve-metal sections aretantalum.

3. The capacitor of claim 1 wherein said nonwetting heatresistantelectrically insulating material is a polymerized fluorocarbon.

4. The capacitor of claim 1 wherein a portion of the electricallyinsulating coating and dielectric layer over the nonporous section areremoved exposing the bare tantalum metal for forming a conductive anodeterminal area of at least a portion of the exposed surface.

5. A method for forming a leadless solid electrolyte capacitorcomprising the steps of:

joining a nonporous valve-metal member and a porous valve metal pelletto form the capacitor anode having a porous and nonporous section;anodizing the anode to form a singular dielectric layer over the surfaceof said sections; coating the portion of the dielectric layer overlyingthe surface of the nonporous section with a nonwetting, heatresistant, telectrically insulating material; depositing a material and saiddielectric layer over said nonporous section for exposing an anodeterminal area in at least a portion of the surface of the nonporoussection

